Biomethanation is a process by which organic matter is decomposed by the simultaneous action of mixed microbial consortia to form methane and carbon dioxide. The microorganisms in these consortia can be classified into three major trophic groups, namely:
I. hydrolytic fermentative bacteria; PA1 II. syntrophic H.sub.2 -producing acetogenic bacteria; PA1 III. H.sub.2 and acetate consuming methanogenic bacteria. PA1 temperature PA1 pressure PA1 pH PA1 support materials PA1 medium composition PA1 microbial composition PA1 essential nutrient requirements PA1 individual dilution rates
The hydrolytic fermentative bacteria of group I can convert a complex organic biomass into simple organic fermentation products including ethanol and organic acids, which are also referred to as volatile fatty acids (VFA's), such as formate, acetate, lactate, propionate, butyrate and benzoate, and hydrogen (H.sub.2) and carbon dioxide (CO.sub.2) gases. The syntrophic H.sub.2 producing bacteria of group II further degrade the organic acids and alcohol into the methanogenic precursors acetate and H.sub.2, and the methanogenic bacteria of group III form methane and carbon dioxide via acetate-cleavage and consume H.sub.2 by reduction of CO.sub.2 to methane.
These trophic groups constitute a complete microbial food chain and they perform some unique functions like the interspecies H.sub.2 -transfer process. The interspecies H.sub.2 -transfer process in a biomethanation-ecosystem is an essential ecophysiological interaction between the H.sub.2 -producing acetogenic bacteria (trophic group II) and H.sub.2 -consuming methanogenic bacteria (trophic group III), where H.sub.2 is an intermediate produced by the trophic group II and it is consumed by trophic group III. H.sub.2 even at very low concentrations inhibits metabolism of trophic group II. Thus, the simultaneous H.sub.2 -consuming activity of methanogenic bacteria in the biomethanation ecosystem (trophic group III) must maintain H.sub.2 -partial pressures low enough to allow growth and metabolism of trophic group II by a so called "syntrophic growth".
Biomethanation has been used in waste treatment to get rid of organic waste.
Prior art anaerobic waste treatment systems (anaerobic contact digestors) used a non-engineered complex microbial ecosystem for waste biomethanation that achieves efficient biological oxygen demand (BOD) removal under anaerobic conditions with methane as a byproduct. However, slow growth of the involved microflora and poor methane productivities make it impossible to operate anaerobic waste treatment plants close to the theoretical biological limits of the ecosystems. Resulting large digestor volumes with high investment costs limit the economic applicability of the technology to high volume waste streams.
The so called "advanced reactor concepts" improve the methane productivities by either recycling of the active bacterial biomass into the process (sludge recycle) and/or the attachment of the active microbial flora to support materials (anaerobic filters). A two stage treatment system where the microbial ecosystem is separated into two populations contained in two different reactors connected by a common liquid phase and operated at acidic pH-values (acidogenic stage) and neutral pH-values (methanogenic stage) has been proposed to improve biomethanation rates. However, the desired methane (CH.sub.4) productivities are rarely obtained. The prior art two stage biomethanation processes do not appear practical because they require high capital costs and two different stages. Upflow-anaerobic-sludge-blanket reactors (UASB-reactors) can render possible high methane productivities because the different microbial trophic groups are associated in discrete microbial granules. However, this system is not used widely because of process instabilities caused by toxic substrates, substrate overloading, nutrient inadequacy and unknown factors which disrupt granulation.
The previously described "advanced reactor concepts" do not involve a microbiological improvement of ecophysiological interactions during the process of biomethanation. The potentially great improvements in biomethanation processes that can be achieved by eco-engineering the physiology of the biocatalysts (i.e. creating the optimum environment for the mixed biocatalyst) is prohibited by present reactor concepts. In prior art processes all the trophic groups are either all placed in one reactor (fixed bed or granules) or in two separate reactors linked by a common liquid phase: the common liquid phase restricts the possibility for either selective improvements in a single reaction step or microbial interactions because the liquid phase is in contact with all the different trophic groups.